A variety of cutting tools are widely used for cutting of metal materials. For example, chemical-vapor-deposition (CVD) tools, physical-vapor-deposition (PVD) tools, cermet tools, cemented carbide tools, and ceramic tools are available and used depending on applications. Of these, CVD tools are tools having coated layers (ceramic coatings) formed on substrates by CVD and PVD tools are tools having coated layers (ceramic coatings) formed on substrates by PVD, whereas cermet tools, cemented carbide tools, and ceramic tools can be classified as tools having no such coated layers.
Recently, CVD tools and PVD tools have increasingly been used with increasing cutting speed. CVD tools are mainly used as turning tools for steel because alumina films, generally having both superior peeling resistance and superior heat resistance, can be formed thereon. PVD tools, on the other hand, are mainly used as milling cutters, which are exposed to large mechanical impacts, because they have a compressive residual stress in the coated layers and therefore have superior breakage resistance.
Nowadays, there is a growing demand for high-speed, high-efficiency machining to improve hourly productivity in the cutting industry, and accordingly the film thickness of CVD tools has been increasing (that is, thicker coated layers have been formed). CVD tools, however, have a problem in that a large tensile residual stress occurs in a coated layer such as an alumina film or a TiCN film due to the difference in coefficient of thermal expansion between the substrate and the coated layer if the film thickness is increased to about 15 μm, thus decreasing the film strength, and also increases the surface roughness of the film, thus decreasing the peeling resistance of the film. Therefore, after the coated layer is formed by CVD, the coated layer is subjected to surface treatment such as blasting or polishing (Japanese Unexamined Patent Application Publication No. 05-116003 (Patent Document 1)). However, it is difficult to release the tensile residual stress of the entire coated layer formed by CVD, and it is therefore difficult to stabilize the breakage resistance of a thick-film CVD tool including a coated layer having a thickness of 15 μm or more.
PVD tools, on the other hand, ensure superior cutting performance in turning applications involving intense mechanical impacts, such as interrupted cutting, because a compressive residual stress can be applied to a coated layer formed by PVD. Accordingly, a cutting tool has been proposed that includes a coated layer whose compressive residual stress distribution is adjusted to improve wear resistance and resistance to chipping (Japanese Unexamined Patent Application Publication No. 2006-082218 (Patent Document 2)). For this proposal, however, it is difficult to form a coated layer having a thickness of 10 μm or more without a film failure because the coated layer has a large compressive residual stress. Accordingly, a PVD tool has been proposed that includes a coated layer formed to a thickness of about 10 μm by PVD so as to have a particular orientation (Japanese Unexamined Patent Application Publication No. 09-323204 (Patent Document 3)). This proposal, however, has a limited range of application because the type of coated layer is limited to one having a particular composition and a particular crystal orientation, and even if the coated layer can be formed without a film failure, a phenomenon by which a compressive failure occurs in the coated layer under an impact upon cutting cannot be sufficiently prevented, with the result that an extended tool life is demanded. In particular, the development of a thick-film PVD tool superior in wear resistance to a CVD tool including a coated layer having a thickness of 15 μm or more has been demanded.